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here's output from a Duplicity backup that I run every night on a server: 

--------------[ Backup Statistics ]--------------
StartTime 1503561610.92 (Thu Aug 24 02:00:10 2017)
EndTime 1503561711.66 (Thu Aug 24 02:01:51 2017)
ElapsedTime 100.74 (1 minute 40.74 seconds)
SourceFiles 171773
SourceFileSize 83407342647 (77.7 GB)
NewFiles 15
NewFileSize 58450408 (55.7 MB)
DeletedFiles 4
ChangedFiles 6
ChangedFileSize 182407535 (174 MB)
ChangedDeltaSize 0 (0 bytes)
DeltaEntries 25
RawDeltaSize 59265398 (56.5 MB)
TotalDestinationSizeChange 11743577 (11.2 MB)
Errors 0
-------------------------------------------------

I don't know if I'm reading this right, but what it seems to be saying is that:

  • I started with 77.7 GB
  • I added 15 files totaling 55.7 MB
  • I deleted or changed files whose sum total was 174 MB
  • My deltas after taking all changes into account totaled 56.5 MB
  • The total disk space on the remote server that I pushed the deltas to was 11.2 MB

It seems to me that we're saying I only pushed 11.2 MB but should've probably pushed at least 55.7 MB because of those new files (can't really make a small delta of a file that didn't exist before), and then whatever other disk space the deltas would've taken.

I get confused when I see these reports. Can someone help clarify? I've tried digging for documentation but am not seeing much in the way of clear, concise plain English explanations on these values.

scooterx3
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1 Answers1

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Disclaimer: I couldn't find a proper resource that explained the difference nor something in the duplicity docs that supports this theory.

ChangedDeltaSize, DeltaEntries and RawDeltaSize do not relate to changes in the actual files, they are related to differences between sequential data. Duplicity uses the rsync algorithm to create your backups which in its turn is a type of delta encoding.

Delta encoding is a way of storing data in the form of differences rather than complete files. Thus the delta changes you see listed is a change in those pieces of data and can therefore be smaller. In fact I think they should be smaller as they are just small snippets of changed data.

Some sources:
- http://duplicity.nongnu.org/ "Encrypted bandwidth-efficient backup using the rsync algorithm" .
- https://en.wikipedia.org/wiki/Rsync " The rsync algorithm is a type of delta encoding.. "
- https://en.wikipedia.org/wiki/Delta_encoding

Edwin
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